This invention deals with the problem of lossy coding of scanned documents, i.e., scanned documents are compressed to a significantly reduced bit rate at the price of a degradation in quality. Current popular approaches, such as that described by L. Bottou et al., High quality document image compression using DjVu, Journal of Electronic Imaging, Vol.7, pp.410-425, Jul. 1998, and R. L. de Queiroz et al., Optimizing block-thresholding segmentation for multiplayer compressing of compound images, IEEE Trans. on Image Processing, Vol.9, pp. 1461-1471, September 2000, both belong to the so-called Mixed Raster Content (MRC)-based approaches. They decompose a document into background, foreground and mask layers, and then compress each layer separately. The principle weakness with such layer-based approach is its intrinsic redundancy. Meantime, the rate control and the scalability feature are not efficiently handled in layer-based approaches.
U.S. Pat. No. 5,778,092, to MacLeod et al. granted Jul. 7, 1998, for Method and apparatus for compressing color or gray scale documents, describes a technique for compressing a color or gray scale pixel map representing a document.
A. Said and A. Drukarev, Simplified segmentation for compound image compression, Proceeding of ICIP 1999, discusses the relative advantages of object-based, layer-based and block-based segmentation schemes.
M. J. Weinberger, The LOCO-I lossless image compression algorithm: principles and standardization into JPEG-LS, IEEE Trans. on Image Processing, Vol.9, No.8, pp.1309-1324, August 2000, describes the LOw COmplexity LOssless COmpression for Images (LOCO-I) compression algorithm.